Inhibition of Cr6+ by the formation of in-situ Cr3+ containing solid-solution in Al2O3–CaO–Cr2O3–SiO2 system

Abstract The pseudo-ternary Al2O3–CaO–Cr2O3 system is of great interest due to various pyrometallurgical applications. However, Cr3+ might be oxidized into toxic and carcinogenic Cr6+ during service, causing harm to humans and the environment. So, doping an adequate amount of SiO2 could suppress the generation of Cr6+. In this experiment, the Al2O3–CaO–Cr2O3 ternary system was doped with different amounts of SiO2 (0–47 wt%) and heat-treated at 1400 °C in the air atmosphere. The reaction mechanism and the phase evolution process were studied by implementing XRD, SEM-EDS, leaching tests, XPS, and associated software. In the Al2O3–CaO–Cr2O3 system under the air atmosphere, chromium primarily exists as a Cr3+ compound CaAl2Cr23+O7 and a Cr6+ compound Ca4Al6Cr6+O16 as a secondary phase. The Cr6+ leaching process was diffusion-controlled and more effective at elevated temperatures. However, Ca4Al6Cr6+O16 disappeared with the addition of SiO2. With the increase of SiO2 content, Cr2O3 tends to be in the Cr3+ form with gehlenite phase (Ca2(Al, Cr)2SiO7) and anorthite phase (Ca(Al, Cr)2Si2O8), thereby inhibiting the Cr6+. Nevertheless, the investigated compositions in the quaternary systems do not exhibit Cr6+ in the air atmosphere at 1400 °C.